System devices including NICs for transmitting/receiving data through a network include, for example, digital multi-function peripherals (hereinafter, MFP), which give/receive image data to/from a host PC. In these MFPs, if no operation is performed or no data transmission is performed by the host PC for a predetermined time period or more, the MFPs are shifted to an energy-saving mode by turning off a fixing heater, a printer controller, etc.
For example, Japanese Laid-Open Patent Publication No. 2003-089254 discloses a technology of driving an input/output portion to execute processes for network monitoring and data transmission/reception such that data can be transmitted/received even when an image forming apparatus is shifted to the energy-saving mode. Japanese Laid-Open Patent Publication No. 2004-110215 discloses a technology of putting an image forming apparatus into the power-saving state if it is detected that all the host computers connected through a network are turned off.
Japanese Laid-Open Patent Publication No. 7-321874 discloses a data communication apparatus that changes a clock frequency of a data processing CPU depending on communication speed to achieve power saving.
However, the conventional arts of Japanese Laid-Open Patent Publication No. 2003-089254 or 2004-110215 do not achieve the power-saving operation of the NIC itself, and the conventional art of Japanese Laid-Open Patent Publication No. 7-321874 only changes the operation clock frequency of the data processing CPU and cannot achieve reasonable power-saving operation corresponding to a plurality of functions included in the NIC.
For example, in the case of the NIC equipped with an encryption chip, although the high-speed processing ability of the encryption chip is effectively utilized in environments where a high-speed encryption process must be executed through link connection at speed of 1 Gbps, it is still problematic that excessive electric power is consumed since the same process is executed and the device is driven at the same level as the 1-Gbps case even if no link exists or a low-speed link such as 10-Mbps link is used.